The output member of the adjustment mechanism is usually fixed directly to the second framework and its axis of rotation is also the axis of pivoting of the backrest.
Because of this, when the backrest of the seat suffers a substantial impact, especially in the event of an accident, the adjustment mechanism may have to withstand extremely high torques and this is especially although not exclusively the case if the seatbelt of this seat is "on board", that is to say if the upper fixing point of this belt is linked to the backrest of the seat.
As a result, in vehicle seats of the prior art, either conventional adjustment mechanisms are used, in which case there may be risks of these mechanisms breaking in the event of an accident, or use is made of adjustment mechanisms which are specially reinforced to withstand high torques, and which have the drawback of being heavier and more expensive than conventional mechanisms.